The microelectronics industry is turning to X-ray lithography (XRL), in order to produce patterns on wafers having minimum feature sizes below the capabilities of optical techniques. It is desired, for example, to reproduce accurately features less than 1 .mu.m. The present invention relates to a process for producing X-ray lithographic masks having such sub-micron features.
It is desired to deposit on a mask membrane an X-ray absorber pattern having a minimum pattern dimension of approximately 0.5 .mu.m. Furthermore the thickness of the absorber, which may be gold, for example, must abe sufficient to produce at least a 5 to 1 X-ray contrast. This is the ratio of transmission through the bare membrane to transmission through the absorber. When the absorber is gold, this means that the minimum absorber thickness must be approximately 5,000 A.
The only available technique for generating sub-micron features with the potential for generating features as small as 0.25 .mu.m is electron-beam lithography. To achieve practical exposure times over large areas, it is necessary to use a sensitive resist such as poly (butene-1 sulfone) (PBS). However, electron scattering limits the thickness of such a resist to approximately 2000-3000 A. It would be desirable to transfer the capability of electron beam lithography to produce fine lines in thin resist layers to thicker resist layers.
As 5,000 A equals 0.5 .mu.m, it will be appreciated that another problem in producing such masks is that they require patterns having width to thickness aspect ratios of approximately 1:1. For various technical reasons known to those skilled in the art, prior art undercut etching techniques for depositing gold in such dimensions by evaporation and liftoff, are not satisfactory.
Another problem presented by such high aspect ratios of the absorber pattern arises from the fact that the mask pattern is exposed on the wafer by means of an X-ray point source. Those portions of the mask which are not in axial alignment with the X-ray point source create undesirable shadowing which it would be preferable to eliminate.
Accordingly, the primary object of the present invention is to provide an improved technique for manufacturing an X-ray lithographic mask having minimum feature sizes less than 1 .mu.m. Another object is to provide such a technique wherein the absorber pattern on the mask results in minimum undesired shading of the X-ray source. Other objects, features, and advantages will become apparent from the following description and appended claims.